1. Hyponatremia (Definition, Types)
2. Causes and Symptoms
3. Role of Tolvaptan
Hyponatremia (which means less blood Sodium concentration) is the most common disorder of hospitalized patients. Normal blood sodium levels is 135-145mq/L; less than 135mq/L is classified as Hyponatraemia. Sodium is the dominant electrolyte in extracellular fluid. The prevalence of hyponatraemia is 2.48% and inadequate management leads to high morbidity and mortality.
Classification
Three (3) types of hyponatraemia;
1. Hypovolemic hyponatraemia (decrease in total body water with greater decrease in total body sodium)
2. Euvolemic hyponatraemia (normal body sodium with increase in total body water; 60% of all cases of hyponatraemia)
3. Hypervolemic hyponatraemia (increase in total body sodium with greater increase in total body water)
Hyponatraemia may also be classified as Acute (developing over 48hrs or less) or Chronic (developing over more than 48hrs) based on duration of episode.
Causes & Symptoms:
Causes of hyponatraemia include ;salt losing nephropathy, renal tubular acidosis,ketonuria,congestive heart failure(CHF), renal impairment, liver cirrhosis, chronic obstructive pulmonary disease(COPD),primary Polydipsia, syndrome of inappropriate antidiuretic hormone secretion(SIADH).
Symptoms may include;
Thirst, muscle cramps/weakness, nausea and vomiting, low urine output, lethargy, headache, confusion, mental status changes, seizures.
Current management options for hyponatraemia has been the use of; fluid restriction, hypertonic saline solution, isotonic solution, urea and loop diuretics.
Role of Tolvaptan:
Tolvaptan is a selective and competitive vasopressin (ADH) receptor antagonist.
Its action involves blocking vasopressin receptors in the vasculature and luminal membranes of collecting ducts. This prevents aquaporins (channel proteins which form pores in membranes to facilitate water transport) insertion into walls which in turn prevents water absorption. This action ultimately results in an increase in urine volume, decrease urine osmolality and increase electrolyte-free water clearance to reduce intravascular volume and an increase in serum sodium levels.
NB: especially useful for heart failure patients as they have high serum levels of vasopressin
Produces significant aquaresis (water excretion without electrolyte excretion) and increase in serum sodium
Elimination half-life (T1/2) is between 6-8hrs.
Indications:
• Symptomatic euvolemic or hypervolemic hyponatraemia secondary to CHF,SIADH,liver cirrhosis
• Hyponatraemia resistant to fluid restriction
• Slowing of kidney decline in patients at risk for rapidly progressing autosomal dominant polycystic kidney disease(ADPKD)
Dosage and Administration:
•Tolvaptan should be initiated and re-initiated in a hospital; Recommended starting dose is 15mg once daily
• Dosage may be increased at intervals ≥24hr to 30mg once daily and to a maximum of 60mg once daily as needed to raise serum sodium.
Contraindications:
• Hypersensitivity to the active substance
• Anuria
• Volume depletion
• Hypernatraemia
• Patients who cannot respond to thirst
• Pregnancy
• Breast-feeding
References:
1. https://reference.medscape.com/drug/samsca-jynarque-tolvaptan-999103
2. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2643096/
3. https://go.drugbank.com/drugs/DB06212
4. https://medlineplus.gov/druginfo/meds/a609033.html
By Pharm.Anthony Osei
Drug Information Unit,KBTH
